How do you find out if you have endometriosis?

Ultrasound is a very useful way to investigate pelvic pain. The most important issue is that the ultrasound needs to be done by a person who is well trained. The best ultrasound specialists are gynaecologists who have specialised in ultrasound (known as COGU. These specialists can be so clever they can see even small deposits of endometriosis. Your gynaecologist will probably have a preferred ultrasound specialist that she trusts. (see my link to safe-endo/ specialist imaging, Ivanhoe)
The only real way to diagnose endometriosis is to have surgery -laparoscopy or keyhole surgery. This is where a gynaecologist puts a camera (laparoscope) into the abdomen/pelvis and looks at the structures of the pelvis. If your gynaecologist sees deposits that look like endometriosis she will cut them out and send them to a pathologist for a formal diagnosis.